New prod uc t al8808, Application information – Diodes AL8808 User Manual
Page 10
AL8808
Document number: DS35648 Rev. 2 - 2
10 of 20
January 2013
© Diodes Incorporated
NEW PROD
UC
T
AL8808
Application Information
(cont.)
Analog Dimming
The CTRL pin can be driven by an external analog voltage (V
CTRL
), to adjust the output current to a value below the nominal average value
defined by R1. The LED current decreases linearly with the CTRL voltage when 0.5V ≤ V
CTRL
≤ 2.5V.
Note that 100% brightness is achieved when either the CTRL pin is left floating or pulled above 2.5V by an external voltage source.
For 2.6V ≤ V
CTRL
≤ 5.5V the LED current will not get overdriven and will be set the current according to the equation V
CTRL
= 2.5V (the internal
reference voltage). See Figure 33 below.
PWM Dimming
LED current can be adjusted digitally, by applying a low frequency Pulse Width Modulated (PWM) logic signal to the CTRL pin to turn the device
on and off. This will produce an average output current proportional to the duty cycle of the control signal. In particular, a PWM signal with a
max resolution of 10bit (~0.1% duty cycle) can be applied to the CTRL pin to change the output current to a value below the nominal average
value set by resistor R
SET
. To achieve this resolution the PWM frequency has to be lower than 500Hz, however higher dimming frequencies can
be used, at the expense of dimming dynamic range and accuracy.
Typically, for a PWM frequency of 500Hz the accuracy is better than 2% for PWM ranging from 5% to 100%.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
V
(V)
Figure 33. LED Current Dimming Ration and
Typical Error vs. Control Voltage
CTRL
110
100
90
80
70
60
50
40
30
20
10
0
D
IM
M
E
D
: 10
0%
L
E
D
C
U
R
R
EN
T
R
A
T
IO
(%)
LED Current Error
Dimmed LED Current Ratio
2.0
2.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
TYPI
C
AL
E
R
R
O
R
(%)
T = 25°C
V = 12V
L = 68µH
A
IN
50 60
0
10 20 30 40
90 100
70 80
PWM DUTY CYCLE (%)
Figure 34. PWM Dimming at 500Hz
0.8
0.7
0.1
0
0.3
0.2
0.6
0.5
0.4
LE
D
C
U
R
R
E
N
T
(A
)
V = 12V
T = +25°C
L = 68µH
R
= 150m
1 LED
IN
A
SET
Ω
V = 12V
T = +25°C
L = 68µH
R
= 150m
1 LED
IN
A
SET
Ω
7.5
0.0
2.5
5.0
15.0
10.0
12.5
PWM DUTY CYCLE (%)
Figure 35. Low Duty Cycle PWM Dimming at 500Hz
L
O
AD
C
U
R
R
E
N
T
E
R
R
O
R
(%)
10
9
8
7
6
5
4
3
2
1
10